Introduction

Rapid eye movement sleep, also called REM sleep or just REM, refers to a period of time during sleep in which the brain suddenly becomes very active and the eyes move rapidly, while an individual is still asleep (Blumberg et al., 2020).

Although sometimes called "dream sleep", REM is not the only sleep stage in which dreams occur (Blumberg et al., 2020).

REM sleep behavior disorder is a sleep disorder in which individuals move and act out their dreams during rapid-eye movement (REM sleep) (Iranzo et al., 2006). It is most often the result of synucleopathies, which are a class of diseases including Parkinson's; however, instances of RBD caused by brain injury have been documented (Iranzo et al., 2006).

In healthy individuals, there isn't a lot of movement during sleep. Our brains "turn off" motor control while we are sleeping, so that we do not thrash around in bed. However, in individuals with RBD, the brain is not able to "turn off" motor control, and so they run, jump, punch, or do whatever it is that they are doing in their dream, in real life.

Why do we need Lesion Network Mapping (LNM)?

Historically, when scientists have been trying to figure out what parts of the brain do what, they have looked at people who experienced a brain injury (such as a blow to the head, a stroke, or anything else that could damage the brain). Then, they would see what symptoms the person had, and where the brain injury was (Fox, 2018).

Scientists would then compare this to a lot of other people with similar symptoms. The idea was that, if everyone who had difficulty speaking (following a brain injury) experienced an injury in the left/front part of their brain, then scientists would be pretty confident that the left/front part of the brain was related to speaking (Fox, 2018).

This was a good technique; however, it was unable to answer some questions (Fox, 2018). For example, say two people both had the SAME symptom following a brain injury. One person had trauma in the right/front part of the brain, the other in the left/back part of the brain. Where was the area of the brain that caused the symptom? It wasn't clear.

A new technique, called lesion network mapping, was created to help solve this problem (Fox, 2018). Here is how it works.

How does LNM Work?

Different parts of the brain are "functionally connected", which basically means that, when performing a task, areas of the brain that are "functionally connected" work together to perform a task. We know if areas A and B in the brain are functionally connected if we can predict the activity level of area A of the brain based on the known activity level of area B (Stephan et al., 2009). A group of brain regions whose activity levels are related to each other is called a network, and the regions in the network work as a team to perform specific brain functions (Fox, 2018).

Scientists have studied large numbers of people to see which areas of their brains were functionally connected. In other words, they studied how people's brains are organized into networks. The project that did this research was called the Human Connectome Project, and it created a kind of map that shows which parts of the brain are connected to which other parts (Fox, 2018; Glasser et al., 2016).

In Lesion Network Mapping, scientists look at the brains of individuals who have experienced a brain injury that resulted in a specific symptom. They compare the physical locations of the brain injuries to the map of brain networks (Fox, 2018). This allows them to see what other areas of the brain are connected to the areas that were injured, via networks, and therefore see which areas of the brain will be affected by the injury, besides just the physical location of the injury.

So, in our earlier example, we had two people with one symptom. One had an injury in the right/front brain, another in the left/back brain. If we just look at what region of the brain is affected, this doesn't make much sense (Fox, 2018). However, using lesion network mapping, perhaps we discover that there is a brain network that includes regions in the right/front brain, the left/back brain, and even some other regions (Fox, 2018). This could lead us to theorize that an injury anywhere on that network, instead of just a specific area, may cause the symptom.

So, in summary, the older technique of lesion-SYMPTOM mapping compares the physical location of the injury to the symptom. Lesion NETWORK mapping compares the affected network to the symptom (Fox, 2018). This helps to explain instances where a brain injury in different locations can create the same symptom.

For a journal article about The Human Connectome (the map of brain networks), see: https://www.nature.com/articles/nn.4361

Generally, brain injuries causing RBD are found in the brainstem, which is the lowest part of the brain, near the base of the skull; however, because RBD is also seen in non-brainstem-related diseases, the brainstem may not be solely responsible for RBD (Iranzo et al., 2006).

More details on what our study found are explained in the "Conclusions" section, at the end of this 10-minute tour.